Method of making lead frame including angle iron tie bar

ABSTRACT

A lead frame and method of making the same are provided. The lead frame includes a die mounting portion, first and second pairs of tie bars, and first and second tie bar bridges extending between respective second extension portions of each tie bar pair. First and second pairs of tie bars are mechanically coupled to respective first and second ends of the die mounting portion. Each of the tie bars includes a first extension portion, a second extension portion, a tie bar span mechanically coupled to the first end of the die mounting portion via the first extension portion, a tie bar flap formed along a longitudinal reinforcement crease, and a lateral reinforcement portion extending from said first extension portion to said die mounting portion. The tie bar flap and the tie bar span lie in intersecting planes and are connected along the longitudinal reinforcement crease between the first extension portion and the second extension portion. The lateral reinforcement portion extends in a direction perpendicular to a direction of said longitudinal reinforcement crease. A first tie bar bridge extends between respective second extension portions of a first tie bar of the first pair of tie bars and a second tie bar of the first pair of tie bars. A second tie bar bridge extends between respective second extension portions of a first tie bar of the second pair of tie bars and a second tie bar of the second pair of tie bars.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a division of U.S. patent application Ser. No.08/909,931, filed Aug. 12, 1997, now U.S. Pat. No. 5,889,318.

BACKGROUND OF THE INVENTION

The present invention relates to fabrication technology used in theassembly of integrated circuit packages and, more particularly, to thedesign of a lead frame for an encapsulated integrated circuit.

According to conventional integrated circuit manufacture, the leadframe, and, in particular, the tie bars of the lead frame, often bows orbecomes distorted during the die attachment and encapsulation process.The result is an improper spatial relationship of the die attach padrelative to the integrated circuit package. Such displacement causesmechanical and electrical failure within the integrated circuit packageand results in loss of system integrity and quality.

Accordingly, a need exists for a lead frame design that effectivelyreduces bowing and distortion of the lead frame during integratedcircuit packaging operations.

BRIEF SUMMARY OF THE INVENTION

This need is met by the present invention wherein a lead frame isprovided and includes an angle iron tie bar with lateral reinforcementportions.

In accordance with one embodiment of the present invention, a lead frameis provided comprising a die mounting portion and at least one tie barmechanically coupled to the die mounting portion. The tie bar includes alongitudinal reinforcement crease defined along at least a portion ofthe tie bar and a tie bar flap formed along the reinforcement crease.The lead frame may further comprise electrically conductive leadsmechanically coupled to the tie bar. The tie bar may further comprises atie bar span defined by the longitudinal reinforcement crease. The tiebar flap may be substantially planar. The tie bar flap and the tie barspan are preferably offset by an angle selected so as to enable closelypacked lead frame stacking, e.g., less than 90°. The die mountingportion and the tie bar preferably form a one-piece, integrallyconstructed, lead frame.

In accordance with another embodiment of the present invention, a leadframe is provided comprising a die paddle and an angle iron tie barmechanically coupled to the die paddle. The angle iron tie bar ispreferably characterized by a bend angle of less than 90°. The angleiron tie bar preferably includes an extension portion, a tie bar spandefined by a longitudinal reinforcement crease, and a tie bar flapformed along the reinforcement crease. The tie bar span may bemechanically coupled to the die paddle via the extension portion.

In accordance with yet another embodiment of the present invention, alead frame is provided comprising a die mounting portion and at leastone tie bar. The tie bar includes a tie bar span mechanically coupled tothe die mounting portion and a substantially planar tie bar flapconnected to the tie bar span. The tie bar span and the tie bar flap maylie in intersecting planes. The angle iron tie bar may include anextension portion and the tie bar span may be mechanically coupled tothe die paddle via the extension portion. The tie bar flap and the tiebar span may be connected along a longitudinal reinforcement creaseformed by a bend in the lead frame.

In accordance with yet another embodiment of the present invention, alead frame is provided comprising a die mounting portion and at leastone tie bar mechanically coupled to the die mounting portion. The tiebar includes a longitudinal reinforcement crease defined along at leasta portion of the tie bar. The tie bar includes a tie bar span and a tiebar flap and the reinforcement crease is formed by bending the tie barflap relative to the tie bar span.

In accordance with yet another embodiment of the present invention, alead frame is provided comprising a die mounting portion and at leastone tie bar mechanically coupled to the die mounting portion. The atleast one tie bar includes a longitudinal reinforcement crease definedalong at least a portion of the tie bar, a tie bar flap formed along thereinforcement crease, and a lateral reinforcement portion extending fromthe tie bar to the die mounting portion. The lateral reinforcementportion extends in a direction perpendicular to a direction of thelongitudinal reinforcement crease and may comprise a chamfered span.

In accordance with yet another embodiment of the present invention, amethod of forming a lead frame is provided comprising the steps of:providing a die mounting portion and at least one tie bar, wherein thetie bar includes a tie bar span mechanically coupled to the die mountingportion; and bending a portion of the tie bar span along a longitudinalreinforcement crease defined along at least a portion of the tie bar soas to form a tie bar flap connected to the tie bar span along thereinforcement crease.

In accordance with yet another embodiment of the present invention, amethod of forming a lead frame is provided comprising the steps of:providing a die mounting portion and at least one tie bar, wherein thetie bar is mechanically coupled to the die mounting portion; and forminga longitudinal reinforcement crease along at least a portion of the tiebar.

In accordance with yet another embodiment of the present invention, amethod of forming a lead frame is provided comprising the steps of:providing a die mounting portion and at least one tie bar, wherein thetie bar includes a longitudinal tie bar span mechanically coupled to thedie mounting portion; and connecting a tie bar flap to the tie bar span.

In accordance with yet another embodiment of the present invention, amounted die arrangement is provided comprising a lead frame and anintegrated circuit die. The lead frame includes a lead frame bodyincluding a plurality of electrically conductive leads, a die mountingportion, and at least one tie bar extending from the lead frame body tothe die mounting portion, wherein the at least one tie bar includes alongitudinal reinforcement crease defined along at least a portion ofthe tie bar and a tie bar flap formed along the reinforcement crease.The integrated circuit die is mounted on the die mounting portion andincludes electrical connections conductively coupled to the electricallyconductive leads. The integrated circuit die may be characterized byphysical characteristics indicative of formation from a wafer includinga plurality of similar integrated circuit dies.

In accordance with yet another embodiment of the present invention, anencapsulated integrated circuit is provided comprising: a plurality ofelectrically conductive leads; a die mounting portion; an integratedcircuit die, at least one tie bar, and an encapsulating material. Theintegrated circuit die is mounted on the die mounting portion andincludes electrical connections conductively coupled to the electricallyconductive leads. The tie bar is mechanically coupled to the diemounting portion and includes a longitudinal reinforcement creasedefined along at least a portion of the tie bar and a tie bar flapformed along the reinforcement crease. The encapsulating materialsurrounds the tie bar, the integrated circuit die, and portions of theelectrically conductive leads to form a solid state encapsulatedintegrated circuit.

In accordance with yet another embodiment of the present invention, amethod of encapsulating an integrated circuit is provided comprising thesteps of: providing a plurality of electrically conductive leads, a diemounting portion, and at least one tie bar mechanically coupled to thedie mounting portion; mounting an integrated circuit die on the diemounting portion, the integrated circuit die including electricalconnections; conductively coupling the electrically conductive leads tothe electrical connection; reinforcing the tie bar by forming alongitudinal reinforcement crease along at least a portion of the tiebar; and encapsulating the integrated circuit die, at least a portion ofthe tie bar, and portions of the electrically conductive leads. The tiebar may include a tie bar flap and a tie bar span and the longitudinalreinforcement crease may be formed by bending a tie bar flap relative toa tie bar span along the longitudinal reinforcement crease.

In accordance with yet another embodiment of the present invention, alead frame is provided comprising a die mounting portion, first andsecond pairs of tie bars, and first and second tie bar bridges extendingbetween respective second extension portions of the tie bar pairs. Eachof the tie bars includes a first extension portion, a second extensionportion, a tie bar span mechanically coupled to the first end of the diemounting portion via the first extension portion, a tie bar flap formedalong a longitudinal reinforcement crease, wherein the tie bar flap andthe tie bar span lie in intersecting planes and are connected along thelongitudinal reinforcement crease between the first extension portionand the second extension portion. A lateral reinforcement portionextends from the first extension portion to the die mounting portion ina direction perpendicular to a direction of the longitudinalreinforcement crease.

Accordingly, it is an object of the present invention to provide a leadframe resistant to bowing and distortion during integrated circuitencapsulation.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is an isometric illustration of a lead frame including angle irontie bar according to the present invention;

FIG. 2 is a cross sectional view of a portion of an angle iron tie bartaken along line 2--2 in FIG. 1;

FIG. 3 is an isometric illustration of an angle iron tie bar providedwith lateral reinforcement portions according to the present invention;

FIG. 4 is an isometric view, partially broken away, of an encapsulatedintegrated circuit incorporating an angle iron tie bar according to thepresent invention; and

FIG. 5 is a plan view of a lead frame including an angle iron tie baraccording to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIGS. 1 and 2, where like structure is represented by likereference numerals, a lead frame 10 according to the present inventionis illustrated in detail. The lead frame 10 comprises a die mountingportion or die paddle 12 having a first end 13A and a second end 13Bopposite the first end 13A. The lead frame 10 also comprises first andsecond pairs of angle iron tie bars 16 mechanically coupled to the diemounting portion 12. Specifically, a first pair of tie bars 16 ismechanically coupled to the first end 13A of the die mounting portion12, and a second pair of tie bars 16 is mechanically coupled to thesecond end of the die mounting portion 12. An integrated circuit die 14is mounted on the die mounting portion 12 to define a mounted diearrangement.

Each tie bar 16 includes a first extension portion 22, a secondextension portion 23, and a tie bar span 17 which is mechanicallycoupled to respective first and second ends 13A, 13B of the die mountingportion 12 via the first extension portion 22. The tie bar span 17 isdefined by a longitudinal reinforcement crease 18. The longitudinalreinforcement crease 18 is defined along at least a portion of the tiebar 16. The tie bar 16 further includes a substantially planar tie barflap 20 formed along the reinforcement crease 18 and connected to thetie bar span 17 between the first extension portion 22 and the secondextension portion 23. The tie bar span 17 and the tie bar flap 20 lie inintersecting planes. Preferably, the die mounting portion 12 and the tiebars 16 form a one-piece, integrally constructed, lead frame 10. A firsttie bar bridge 32 extends between respective second extension portions23 of a first pair of tie bars 16. Similarly, a second tie bar bridge 34extends between respective second extension portions of the second pairof tie bars 16. As is clearly illustrated in FIGS. 1 and 2, each tie barflap 20 is connected to the remainder of the lead frame 10 exclusivelyalong the reinforcement crease 18, i.e., there are no additionalintegral connections between the tie bar flap 20 and the remainder ofthe lead frame 10.

Preferably, the tie bar flap 20 and the corresponding tie bar span 17and longitudinal reinforcement crease 18 are formed by a bend in thelead frame 10. Specifically, the reinforcement crease 18 is formed bybending the tie bar flap 20 relative to the tie bar span 17. It iscontemplated by the present invention, however, that the tie bar flap 20and the tie bar span 17 may be provided in a manner other than bending.For example, the tie bar flap 20 may be welded to the tie bar span 17.Further, it is contemplated by the present invention that the tie barflap 20 may be connected to the tie bar span 17 at a location other thanthe edge of the tie bar span 17.

Each tie bar flap 20 and corresponding tie bar span 17 are preferablyoffset by a bend angle θ of less than 90° to enable closely packedstacking of a plurality of lead frames 10, see FIG. 2. Specifically,closely packed stacking is achievable at bend angles θ less than 90°because the base edges 21 of the tie bar flaps 20 fit within the tie barseparation space 19 when a lead frame 10 is stacked upon another leadframe 10. At bend angles close to or above 90°, the base edges 21 of thetie bar flaps 20 abut the tie bar span 17 of the other lead frame 10 andclosely packed spacing is not achievable without thick and expensivelead frame separation material.

Referring to FIGS. 4 and 5, a lead frame 100 comprises a lead frame body110 including a plurality of electrically conductive leads 26, the diemounting portion 12, and the tie bars 16. The integrated circuit die 14includes electrical connections 15 conductively coupled to the leads 26.Each tie bar 16 extends from the lead frame body 110 to the die mountingportion 12. As is noted above with reference to FIGS. 1 and 2, each tiebar 16 includes the longitudinal reinforcement crease 18 defined alongat least a portion of the tie bar 16 and a tie bar flap 20 formed alongthe reinforcement crease 18. The leads 26 are mechanically coupled tothe tie bar 16 via the lead frame body 110.

Portions of the lead frame body 110 are removed, as indicated by thehatched line 25, after encapsulation of the lead frame 100 and the die14. An encapsulated integrated circuit 28 is illustrated in FIG. 4 andcomprises an encapsulating material 29 surrounding the tie bars 16, theintegrated circuit die 14, and portions of the electrically conductiveleads 26. The encapsulating material 29 physically binds the integratedcircuit die 14 and forms a solid state encapsulated integrated circuit28.

For the purposes of describing and defining the present invention, itshould be appreciated that the integrated circuit die 14 typicallycomprises a patterned-substrate integrated circuit cut from asemiconductor wafer including a plurality of similar integratedcircuits. However, it should also be appreciated that an integratedcircuit die, as utilized herein, is not limited to integrated circuitsformed from a wafer of dies. Rather, the integrated circuit die 14comprises an integrated circuit formed on a substrate. It iscontemplated by the present invention that a "lead frame," as referredto in the present description and claims, does not necessarilyincorporate electrically conductive leads. Rather, the lead frameaccording to the present invention may merely serve to support a die ora die paddle mechanically.

Referring now to FIG. 3, an alternative embodiment of the presentinvention, including an angle iron tie bar 10' provided with lateralreinforcement portions 30, is illustrated. The lead frame 10' comprisesthe die mounting portion or die paddle 12 and respective pairs of tiebars 16' mechanically coupled to opposite ends of the die mountingportion 12. Each tie bar 16' includes the longitudinal reinforcementcrease 18 and the tie bar flap 20. In addition, lateral reinforcementportions 30 extend from the each tie bar 16' to the die mounting portion12 in a direction perpendicular to a direction of the longitudinalreinforcement crease 18. The lateral reinforcement portion 30 comprisesa chamfered span, i.e., a lead frame portion bounded by a diagonalprojection from the tie bar 16 to the die mounting portion 12. Thelateral reinforcement portion 30 is operative further to reduce bowingand distortion of the tie bar 16 during encapsulation.

Having described the invention in detail and by reference to preferredembodiments thereof, it will be apparent that modifications andvariations are possible without departing from the scope of theinvention defined in the appended claims.

What is claimed is:
 1. A method of forming a lead frame comprising thesteps of:providing a die mounting portion and at least one tie bar,wherein said tie bar includes a tie bar span mechanically coupled tosaid die mounting portion; and bending a portion of said tie bar spanalong a longitudinal reinforcement crease defined along at least aportion of said tie bar so as to form a tie bar flap connected to saidtie bar span along said reinforcement crease.
 2. A method of forming alead frame comprising the steps of:providing a die mounting portion andat least one tie bar, wherein said tie bar includes a longitudinal tiebar span mechanically coupled to said die mounting portion; andconnecting a tie bar flap to said tie bar span.
 3. A method ofencapsulating an integrated circuit comprising the steps of:providing aplurality of electrically conductive leads, a die mounting portion, andat least one tie bar mechanically coupled to said die mounting portion;mounting an integrated circuit die on said die mounting portion, saidintegrated circuit die including electrical connections; conductivelycoupling said electrically conductive leads to said electricalconnection; reinforcing said tie bar by forming a longitudinalreinforcement crease along at least a portion of said tie bar, whereinsaid tie bar includes a tie bar flap and a tie bar span and wherein saidlongitudinal reinforcement crease is formed by bending said tie bar flaprelative to said tie bar span along said longitudinal reinforcementcrease; and encapsulating said integrated circuit die, at least aportion of said tie bar, and portions of said electrically conductiveleads.
 4. A method of forming a lead frame as claimed in claim 1 whereinsaid tie bar flap is connected to said lead frame exclusively along saidreinforcement crease.
 5. A method of forming a lead frame as claimed inclaim 2 wherein said tie bar flap is connected to said lead frameexclusively along said reinforcement crease.